The present invention relates to a microwave responder which resonates or does not resonate with a microwave in accordance with a response signal, radiates the resonated microwave, and generates a harmonic component upon resonance to radiate a harmonic signal wave which is amplitude-modulated by the response signal.
In recent years, there is proposed a communication system in which a responder is carried by a person or is attached to a mobile object, proper information is stored in the responder, a microwave is transmitted to the responder from a stationary radio transmitter, and the responder receives the microwave and sends a response signal wave modulated by a response signal. Depending on information to be stored in the responder, the responder can serve as an ID card, a driver's license, a specification instruction manual in a production line, and the like. Since the communication system uses the received microwave as a carrier wave of the response signal wave, the responder itself does not require an oscillation circuit, and power consumption of the responder can be very small. Thus, when an electric power of the received microwave is converted to a DC power to be used as a driving power source, the responder itself need not incorporate a battery or need not be externally supplied with a commercial AC power source through cables.
FIG. 9 shows an example of a conventional microwave responder which receives a microwave, phase-modulates it by a response signal, and sends back the modulated microwave as a response signal wave in the communication system as described above.
In FIG. 9, a rectangular microstrip resonator 1 whose one side has a length corresponding to 1/2 a wavelength .lambda. of a microwave to be received is arranged on a low-dielectric substrate (not shown) on the lower surface of which a ground plate is disposed. A microstrip line 2 having a length larger than the 1/2 wavelength is connected to the central portion of the one side of the resonator 1. A short-circuiting stub 4 is connected through a transistor 3 to a position of the microstrip line 2 corresponding to the 1/2 wavelength from its free end, so that an ON/OFF state of the transistor 3 is switched in response to a response signal.
In this arrangement, when the transistor 3 is OFF, a microwave received by the microstrip resonator 1 is reflected by the free end of the microstrip line 2 and is radiated again from the microstrip resonator 1. When the transistor 3 is ON, the microstrip line 2 is, in effect, short-circuited at the position where the transistor 3 is connected. The microwave is reflected at this position, and is radiated again from the microstrip resonator 1 while the phase of the microwave is shifted by 90.degree. . In this manner, the effective length of the microstrip line for reflecting the received microwave is switched by a response signal, and the received microwave is phase-modulated and is sent back as a response signal.
In the conventional microwave responder shown in FIG. 9, the microstrip resonator 1 and the microstrip line 2 are connected to each other, and the total size must be larger than one wavelength of a microwave to be received, i.e., a large space is required. Thus, a demand has arisen for a compact responder.
The response signal wave sent back from the responder has a very weak electric field intensity, and is easily influenced by fading or noise. Thus, a system which can discriminate whether or not a response signal can be accurately transmitted is demanded. For this purpose, the same response signal is transmitted by a carrier wave having a frequency different from that of the response signal wave, and two demodulated response signals transmitted by the two different carrier waves are compared. If a coincidence between the two signals is detected, it can be determined that the response signal wave is not influenced by, e.g., fading. As the carrier wave having a frequency different from that of the response signal wave, a harmonic component corresponding to an integer multiple of a microwave may be used. In this case, the harmonic component is amplitude-modulated by a response signal, and is radiated as a harmonic signal wave. However, the conventional microwave responder itself cannot radiate a harmonic component, and a means for generating a harmonic signal wave must be separately added.